Use HTML beautifier to indent ERB files

We had inconsistent indentation in many places. Now we're fixing them
and adding a linter to our CI so we don't accidentally introduce
inconsistent indentations again.

Gemfile

@@ -95,6 +95,7 @@ group :development do
   gem "capistrano3-puma", "~> 5.2.0"
   gem "erb_lint", "~> 0.9.0", require: false
   gem "faraday-retry", "~> 2.2.1", require: false
+  gem "htmlbeautifier", "~> 1.4.3", require: false
   gem "mdl", "~> 0.13.0", require: false
   gem "pronto", "~> 0.11.2", require: false
   gem "pronto-erb_lint", "~> 0.1.6", require: false